The present invention is directed to stretch wrap films and methods for their manufacture. In particular, the present invention is directed to stretch wrap films having excellent maximum stretch, moderate holding force, superior puncture resistance, high total energy dart drop, high cling force and overall strength.
The use of thermoplastic stretch wrap films for the overwrap packaging of goods, and in particular, the unitizing of palleted loads is a significant commercially important application of polymer film, including generically, polyethylene and other polyolefins.
Over-wrapping a plurality of articles to provide a unitized load can be achieved by a variety of techniques. In one procedure, the load to be wrapped is positioned on a platform, or turntable, which is made to rotate and in so doing, to take up stretch wrap film supplied from a continuous roll. Braking tension is applied to the film roll so that the film is continuously subjected to a stretching or tensioning force as it wraps around the rotating load in overlapping layers. Generally, the stretch wrap film is supplied from a vertically arranged roll positioned adjacent to the rotating pallet load. Rotational speeds of from about 5 to about 50 revolutions per minute are common.
At the completion of the overwrap operation, the turntable is completely stopped and the film is cut and attached to an underlying layer of film employing tack sealing, adhesive tape, spray adhesives, etc. Depending upon the width of the stretch wrap roll, the load being overwrapped can be shrouded in the film while the vertically arranged film roll remains in a fixed position. Alternatively, the film roll, for example, in the case of relatively narrow film widths and relatively wide pallet loads, can be made to move in a vertical direction as the load is being overwrapped whereby a spiral wrapping effect is achieved on the packaged goods.
Some of the properties desired of a good stretch wrap film are as follows: good cling or cohesion properties, high puncture resistance, good machine direction tear resistance, good transparency, low haze, low stress relaxation with time, high resistance to transverse tear especially when under machine direction tension, can be produced in thin gauges, low specific gravity and thus high yield in area per pound, good tensile toughness, high machine direction ultimate tensile strength, high machine direction ultimate elongation, and low modulus of elasticity.
Currently, different grades of stretch wrap films are commonly marketed for different end uses according to overall film properties. For example, certain stretch wrap films have superior properties for load retention, but these films are characterized by having poor stretching characteristics. On the other hand, certain stretch wrap films having superior stretching properties have low load retention properties, thus limiting their use. Some of the broader categories include: general purpose stretch films, premium or heavy duty stretch films, single sided cling films, general purpose hand wrap films, heavy duty hand wrap films, and special formulation hand wrap films.
A need exists to develop superior stretch wrap films characterized by having heavy load retention characteristics and excellent stretching characteristics while still maintaining other important stretch film properties. Such films could be used in a wider variety of end applications and, thus, not unduly limit users of stretch wrap films to selectively choosing a film based on its properties prior to initiating a stretch wrap application.
Accordingly, the present invention provides for multi-layer films for premium stretch applications including stretch wrap films. The preferred resultant film has five layers with each layer being a single component or blended components to achieve the desired improved results.
In accordance with the present invention, attention has been focused on developing higher performance stretch films, while maintaining cost efficient production capabilities. Performance enhancements include improvement of load retention, tear resistance in machine direction (MD) and transverse direction (TD), puncture resistance, ultimate elongation, and overall strength. Desired properties of a xe2x80x9cgoodxe2x80x9d stretch film are good cling or cohesion properties, good tear resistance in MD and TD directions, good clarity (low haze), high ultimate elongation, high tensile values, good stiffness (modulus), and high yield per pound.
The present invention is a co-extruded film comprised of a 5-layer construction that is used for cling/cling film applications or premium grade stretch film applications. In one embodiment, these multi-layer stretch wrap films are manufactured as cast films with conventional co-extrusion methods. Each of the five layers comprises a mixture of polyolefin polymers. When a mixture of polymers is utilized in a single layer, the polymers are introduced into an extruder to be mixed and extruded as a single layer. Cling/cling or premium grade stretch film application construction comprises the utilization of at least five layers of the following compositions.
In accordance with one embodiment, the present invention provides a multi-layer film comprising at least five layers. A first outer layer comprises a blend of linear low density polyethylene, a metallocene catalyzed linear low density polyethylene, an ethylene plastomer and an ethylene/methyl acrylate copolymer. A first inner layer comprises a blend of linear low density polyethylene and a metallocene catalyzed linear low density polyethylene. A core layer comprises a blend of linear low density polyethylene and polypropylene. A second inner layer comprises a blend of linear low density polyethylene and a metallocene catalyzed linear low density polyethylene. The second inner layer may be the same or different than the first inner layer. A second outer layer can have the same or a different formulation as the first outer layer. The second outer layer comprises a blend of linear low density polyethylene, a metallocene catalyzed linear low density polyethylene, an ethylene plastomer and an ethylene/methyl acrylate copolymer.
In accordance with one embodiment, the present invention is directed to a multi-layer film comprising at least two outer layers, two inner layers and a core layer. A first outer layer represents from 8 to 25 percent by weight of the film and comprises a blend of linear low density polyethylene, a metallocene catalyzed linear low density polyethylene, an ethylene plastomer and an ethylene/methyl acrylate copolymer. A first inner layer represents from 15 to 35 percent by weight of the film and comprises a blend of linear low density polyethylene and a metallocene catalyzed linear low density polyethylene. A core layer represents from 15 to 35 percent by weight of the film and comprises a blend of linear low density polyethylene and polypropylene. A second inner layer represents from 15 to 35 percent by weight of the film and comprises a blend of linear low density polyethylene and a metallocene catalyzed linear low density polyethylene. A second outer layer represents from 8 to 25 percent by weight of the film and comprises a blend of linear low density polyethylene, a metallocene catalyzed linear low density polyethylene, an ethylene plastomer and an ethylene/methyl acrylate copolymer. The second outer layer is optionally different than the first outer layer and comprises a linear low density polyethylene.
In accordance with one embodiment of the present invention, a multi-layer film comprises a first outer layer representing from 8 to 25 percent by weight of the film and comprising a blend of 20 to 40 percent by weight of a linear low density polyethylene octene copolymer, 25 to 65 percent by weight of a low polydispersity linear low density polyethylene octene copolymer, 1 to 15 percent by weight of an ethylene plastomer having a melt index from 3.5 to 5.5 g/10 min. and a density of from 0.871 to 0.875 grams per cubic centimeter, and 10 to 30 percent by weight of an ethylene/methyl acrylate copolymer. The film also comprises a first inner layer representing from 15 to 35 percent by weight of the film and comprising a blend of from 50 to 95 percent by weight of a linear low density polyethylene octene copolymer, and from 5 to 50 percent by weight of a metallocene catalyzed linear low density polyethylene.
A core layer representing from 15 to 35 percent by weight of the film comprises a blend of from 60 to 90 percent by weight of a linear low density polyethylene octene copolymer, and from 10 to 40 percent by weight of a propylene ethylene random copolymer. A second inner layer representing from 15 to 35 percent by weight of the film and comprises a blend of from 50 to 95 percent by weight of a linear low density polyethylene octene copolymer, and from 5 to 50 percent by weight of a metallocene catalyzed linear low density polyethylene.
A second outer layer representing from 8 to 25 percent by weight of the film and comprises a blend of 20 to 40 percent by weight of a linear low density polyethylene octene copolymer, 25 to 65 percent by weight of a low polydispersity linear low density polyethylene octene copolymer, 1 to 15 percent by weight of an ethylene plastomer having a melt index from 3.5 to 5.5 g/10 min. and a density of from 0.871 to 0.875 grams per cubic centimeter, and 10 to 30 percent by weight of an ethylene/methyl acrylate copolymer.